Fe-Cr System Research Articles

Influence of hydrogen on the elastic properties and dislocation behavior in Fe–Cr and Fe–Ni alloys by DFT calculations

The effect of interstitial hydrogen on the elastic properties of bcc Fe, bcc Fe–Cr, and bcc Fe–Ni was investigated using density functional theory calculations. Our results indicate that the elastic moduli decrease linearly with increasing hydrogen concentration. The consequences of hydrogen for the mechanical properties of bcc Fe, bcc Fe–Cr, and bcc Fe–Ni were analyzed, considering various factors such as the ideal shear stress, Peierls stress, number of dislocation pile-ups, and critical crack growth lengths. At the same hydrogen concentration, compared to the bcc Fe and bcc Fe–Ni systems, fewer dislocation pile-ups and shorter critical crack growth lengths can facilitate the nucleation and propagation of cracks in the bcc Fe–Cr system. Finally, we propose a mechanism to explain the influence of Cr and Ni on hydrogen embrittlement.

Influence of injected ions on α’ formation under ion irradiation in Oxide Dispersion Strengthened Steels

Oxide Dispersion Strengthened (ODS) steels hold great promise for applications in next generation reactors. Under irradiation, a phase separation α/ α’ can occur within the Fe-Cr matrix of ODS steels that can alter their mechanical properties. This work presents, for the first time, the characteristics of α’ precipitates enhanced by ion irradiation at 400 °C and examines the influence of the implanted ions. Far from the implanted region, α’ is reported in significant density while at the implanted peak, the α’ density is considerably reduced. This suggests that ion implantation either reduces the fraction of α’ phase formed after irradiation or delays considerably its formation. Through atom probe tomography analysis and comparison with existing literature, the low impact of the damage rate and fluence on the α’ formation in ODS steels is highlighted. Interestingly, the efficiency of ballistic mixing of α’ appears to be less pronounced in ODS steels than in Fe-Cr systems.

Effect of Ce on Inclusions and Precipitates in 10Cr5MoV Steel and the Microscopic Behavior of Solid Solution Ce in Steel

In this article, the effects of Ce (0–120 ppm) on inclusions and precipitates in industrially produced CO2/H2S‐resistant 10Cr5MoVCe special oil well pipe steel and the microscopic behavior of Ce solid solution in steel were systematically studied by scanning electron microscopy, transmission electron microscopy, electrolysis separation of nonaqueous mixed solutions, and first‐principles calculations. After adding Ce to 10Cr5MoV steel the inclusions changed from chain‐like Al2O3 inclusions to polygonal Ce–O–S–Al–Ca complex inclusions and spheroidal Ce2O3, CeAlO3, Ce2O2S, and CeS inclusions. When the total Ce content was 25–120 ppm, the solid solution Ce content was maintained at about 20% of the total Ce content. The Ce had no significant influences on the types of chromium carbide precipitates, and the chromium carbide precipitates of all tested steels were Cr23C7 and Cr7C3. However, with the increase of Ce, the content of Cr23C7 and Cr7C3 precipitates decreased slightly, from 1.27% without Ce to 1.18% when Ce was 120 ppm. The first‐principles calculation results show that there was strong interaction between Fe, Ce, and Cr. Ce enhanced the interaction force between the Fe and Cr atoms, and improved the stability of the system, which facilitated the solid solution of Cr in the Fe–Cr system.

DETERMINATION OF THE THERMODYNAMIC PARAMETERS OF α-IRON IN BINARY SYSTEMS Fe-Cr AND Fe-Ti

This article presents the results of calculations of the thermodynamic parameters of binary Fe-Cr and Fe-Ti systems based on their state diagrams using the Bjerrum-Guggenheim osmotic coefficient, for the region of iron crystallization from the iron activity of an ideal α-γ solution for the above systems. When calculating the lines of phase equilibrium in the Fe-Cr and Fe-Ti systems, it was established that the Bjerrum-Guggenheim coefficient can serve as an assessment parameter characterizing the role of exchange forces between atoms, and by its dimension (Фi><1) one can qualitatively judge the nature of the intermolecular interactions in the system. The results of this work made it possible to correctly solve the direct Gibbs problem, namely, to obtain an analytical dependence of phase compositions on temperature at phase equilibria based on the laws of phase formation. It is shown that with the help of the Bjerrumm-Guggenheim osmotic coefficient (Fi), the real equilibrium in the system under study can be determined in detail and their correct analytical expression can be obtained. When calculating phase equilibrium lines in systems with different types of interaction between components, it was established that the Bjerrum-Guggenheim coefficient can serve as an assessment parameter characterizing the role of exchange forces between atoms. Keywords: thermodynamics; Bjerrum-Guggenheim coefficient; state diagram; osmotic coefficient; binary systems.

A novel low-cost Ti–Cr–Fe β-Ti alloy with TWIP effect: Sensitivity of tensile properties to microstructural characteristics

A novel low-cost Ti-4.6Cr–3Fe β-Ti alloy with twining induced plasticity effect was designed, and its tensile properties and deformation mechanisms were sensitive to its microstructural characteristic. The received sample with the microstructure of β matrix embedded isothermal ω (ωiso) precipitations exhibited the largest tensile strength (Rm), but the limited ductility (<3 %). When the solution treatment (ST) temperature increased from below to above α→β transition temperature, the microstructure changed from β matrix with αs precipitations to β matrix with athermal ω (ωath) precipitations, and the deformation mechanisms correspondingly changed from dislocation slip to the combination of dislocation slip and {332} deformation twins. Among different treatment samples, the ST800 sample obtained a relatively good combination of Rm (1146 MPa) and elongation to failure (EL, 12.5 %). It is surprising that, as the ST temperature increased from 800 °C to 900 °C, the Rm increased from 1146 MPa to 1230 MPa, but the fracture mode changes from ductile fracture to brittle fracture due to the formation of Laves phase (TiFe2) at grain boundaries. These findings provide a basis for improving the tensile properties of Ti-4.6Cr–3Fe alloy by adjusting the microstructure, and a reference for optimizing the composition of Ti–Cr–Fe system low cost β-Ti alloys.

Determination of the Curie temperature of structural alloys by analyzing the electrical characteristics of wires

The Curie temperature for many structural multicomponent alloys can only be estimated approximately, for example, from state diagrams. The necessity of an operational assessment of this parameter arises in the design of technologies for processing and controlling structural materials, in mathematical modeling of physical processes. The paper describes an experimental method for determining the temperature of magnetic transformation by measuring the temperature dependences of inductance and electrical resistance of wire samples. Verification of the procedure was carried out by determining the Curie temperature of nickel and comparing it with data obtained by other methods. We established the dependence of the Curie temperature of nickel on the carbon content and determined the Curie temperatures for alloys of Fe-Cr and Fe-Ni systems. Using numerical modeling and experimental data, temperature dependences of the relative magnetic permeability of materials were obtained. The results show the presence of the Hopkinson effect in the materials under study and the presence of inflection points on the electrical resistance curves.

Effect of thermal treatment of chromium iron melts on the structure and properties of castings

The article describes the results of experimental studies for the effect of thermal treatment (TTM) of G-X300CrMo27-1 high-chromium cast iron samples in the solid–liquid and liquid state on the structure, phase composition, and properties of ingots. For ingots with a carbon content of 2.8 to 4.5 wt%, cooled at a rate of 3.3 * 10−2 deg s−1, the dependencies of the structure, phase composition, composition of primary, eutectic carbides and matrix, hardness HV and microhardness of its individual phases and resistance to abrasive wear from the temperature of isothermal holding at TTM. A significant effect of the TTM temperature of melts on the structure and properties of high-chromium cast iron ingots was detected. The temperatures of inflection on the plotted curves for the characteristics of the structure and properties of the ingots were associated with a phase transition in iron at 1400 °C and with a point on the phase diagram liquidus of the Fe-Cr system. This as well as an increase in the concentration of chromium in the composition of primary carbides with an increase in the TTM temperature up to 1480 °C, made it possible to assume that (Cr, Fe)7C3 clusters stable in composition were formed in the melt of such cast irons below ∼1500 °C. The reasons for the decrease in the size of primary carbides during G-X300CrMo27-1 molten chromium iron overheating above 1500 °C were substantiated based on these data. During the TTM of fine-crystalline ingots made of hypereutectic cast iron in the temperature range between the liquidus and solidus lines, it was detected that the primary carbides (Cr, Fe)7C3 recrystallized resulting in a significant decrease in the chromium content and an increase in the iron content in them. At the same time, their sizes did not change significantly. Their share increased, and the share of eutectic carbides decreased. Such a process also had a significant impact on the properties of the resulting ingots. The most preferable temperatures of hot metal cast irons in the liquid and solid–liquid states were identified based on the results of the studies.

Kinetics of Transformation, Border of Metastable Miscibility Gap in Fe–Cr Alloy and Limit of Cr Solubility in Iron at 858 K

The study was aimed at determination of the position of the Fe-rich border of the metastable miscibility gap (MMG) and of the solubility limit of Cr in iron at 858 K. Towards this end, a Fe73.7Cr26.3 alloy was isothermally annealed at 858 K in vacuum up to 8144 hours and Mössbauer spectra were recorded at room temperature after every step of the annealing. Three spectral parameters viz. the average hyperfine field, 〈B〉, the average isomer shift, 〈IS〉, and the probability of the atomic configuration with no Cr atoms in the two-shell vicinity of the probe Fe atoms, P(0,0), gave evidence that the transformation process took place in two stages. All three parameters could have been well described in terms of the Johnson–Mehl–Avrami–Kolmogorov equation, yielding kinetics parameters. The first stage, associated with the phase decomposition, proceeded much faster than the second stage, associated with the alpha-to-sigma phase transformation. The most reliable estimation of the position of the MMG and that of the value of the Cr solubility limit was obtained from the annealing time dependence of 〈B〉, namely 24.5 at. pct Cr for the former and 20.3 at. pct Cr for the latter. A comparison of these figures with the recent phase diagrams pertinent to Fe–Cr system was done.

Effect of precursor density on the wear resistance of in-situ TiC/Fe matrix composites based on Fe–Cr system moderator

In this study, the effects of precursor density on the size, distribution, and volume fraction of in-situ TiC particles were investigated through an analysis of high chromium cast iron (HCCI) matrix composites reinforced with in-situ TiC particles which were made through casting infiltration. The microstructure of different composites was observed, and their mechanical properties and wear resistance were analyzed. The results showed that with the increase of the precursor density from 2.954 g cm−3 to 3.546 g cm−3, the average size of the TiC particles grew from 0.19 μm to 0.66 μm, while their volume fraction decreased from 69.75% to 50.89%, and the compressive strength of the composites fell from 533 MPa to 428 MPa. When the precursor density was 2.954 g cm−3, the composites exhibited the best wear resistance, which was 1.98 times as good as those with the precursor density of 3.546 g cm−3. The wear mechanism of the abrasives in the composites lies in their cutting effect and the protection of TiC particles. The existence of a body of submicron TiC particles enhanced the deformation resistance of the matrix as they were able to reduce wear. This study contributes to the research of wear-resistant materials and enables them to be widely applied in the future.

Nucleation-Growth Versus Spinodal Decomposition in Fe-Cr Alloys: An Experimental Verification by Atom Probe Tomography and Small Angle Neutron Scattering.

Identifying the operative mode of phase separation [spinodal decomposition (SD) or nucleation-growth (NG)] remains an extremely important area of research. The present work examines this critically in the Fe-Cr system using atom probe tomography (APT) and small angle neutron scattering (SANS), and establishes the framework to distinguish the two different modes of α' phase separation in thermally aged Fe-35 at% Cr and Fe-20 at% Cr alloys. Independent APT analysis determines the mode of phase separation on the basis of (i) the presence/absence of periodic chemical fluctuation through radial distribution function analysis and (ii) interphase interface characteristics (diffuse/sharp). SANS analysis, in contrast, yields virtually indistinguishable correlation peaks for both the modes, which necessitates further investigation of the several different aspects of SANS profiles in the light of APT results. For the first time, key features of SANS profiles have been identified that can unambiguously distinguish SD from NG in the Fe-Cr system: (i) nature of temporal evolution of FWHM of the correlation peak and (ii) appropriate value of γ for fitting with the dynamic scaling model (γ = 6 for SD, Fe-35 at% Cr alloy; γ = 4 for NG, Fe-20 at% Cr alloy).

Thermodynamic assessment of the Co–Cr–Fe system and atomic mobility study of its fcc phase

In the present work, the Co–Cr–Fe system was thermodynamically assessed by using the CALPHAD approach coupled with first-principles calculations and experimental data from the literature. Subsequently, the fcc Co–Cr–Fe ternary diffusion couples annealed at 1273 and 1473K were scanned by using electron probe microanalysis (EPMA) to obtain the composition profiles, from which the interdiffusion coefficients were extracted by the Whittle-Green method. Based on these interdiffusion coefficients and present thermodynamic parameters, the atomic mobilities of Co, Cr, and Fe in the fcc Co–Cr–Fe alloys were assessed by means of DICTRA software. The calculated interdiffusion coefficients, composition profiles and diffusion paths of the fcc Co–Cr–Fe alloys were consistent with the experimental data, which verifies the accuracy of the assessed atomic mobilities.

СРАВНЕНИЕ РЕЗУЛЬТАТОВ ТЕРМОДИНАМИЧЕСКОГО И ПЕРВОПРИНЦИПНОГО МОДЕЛИРОВАНИЯ НЕУПОРЯДОЧЕННЫХ РАСТВОРОВ СИСТЕМЫ Fe–V

This paper considers the concentration dependences of the average magnetic moment of the Fe–V system and solution enthalpy as a function of the vanadium concentration from 1 to 10 at% for a disordered Fe–V solid solution in an orderly magnetic state using first-principle calculations of the electronic structure in the WIEN2k software package. The results are compared with data obtained using CALPHAD-models in previous studies. A conclusion about the prospects of finding a possible inversion of the short-range order in Fe–V alloys similar to the inversion in Fe–Cr systems has been made on this basis.

Phase Separation Behavior of Fe–Cr System Including α, α′, and σ Phases Using Phase-Field Modeling

We suggested the multi-phase phase-field model of Fe–Cr system with consideration of not only α-α′ but also σ phase. In the prior phase-field study, one assumes the specific mechanism of phase separation, i.e., spinodal decomposition or nucleation and growth. On the other hand, our phase-field model can concurrently consider spinodal decomposition or nucleation/growth without any assumptions. With developed phase-field model of Fe–Cr system, we modelled three types of two-phase structures: α-α′, α-σ, α′-σ and single-phase σ region at T = 700 K and 1000 K.

Macroscopic magnetic hardening due to nanoscale spinodal decomposition in Fe–Cr

The Fe–Cr alloy system is the basis of ferritic steels, which are important structural materials for many applications, including their use in future fusion reactors. However, when exposed to elevated temperatures and radiation, the Fe–Cr system can undergo phase separation, resulting in Fe-rich (α) and Cr-rich (α’) nanoscale regions. This in turn generates the so-called “475 °C embrittlement” and modifies the magnetic properties. The correlation between the microstructural and magnetic changes is however poorly understood, which currently prevents the possibility of assessing the material in a non-destructive way by magnetometry. Here, we study the microstructural decomposition of an Fe–40Cr alloy induced by annealing at 500 °C for extensive time scales and its impact on the magnetic properties using magnetometry and advanced experimental methods, such as atom probe tomography, transmission electron microscopy (TEM), and micromagnetic simulations. Upon annealing, the alloy rapidly exhibits a spinodal decomposition morphology with a typical length scale of about 10 nm. With increasing annealing time, the hardness assessed by Vickers testing, the magnetic saturation, and the coercivity increase, which correlates with an increase in α-volume fraction and the system's heterogeneity. The magnetic domain patterns imaged by TEM and interpreted with the help of micromagnetic simulations reveal at the nanometer scale the impact of decomposition on the magnetic response of Fe–Cr.

New method of atomistic modeling of [formula omitted] phase transition in Fe–Cr alloy with effective accounting for vibrational entropy

In this work, a novel approach was developed for studying the early stages of the formation of Cr-rich precipitates (α′ phase) in Fe–Cr system. The approach accounts for vibrational entropy of the system. The research was conducted using machine-learning potentials. The potentials have a form of low-rank tensors. Two such potentials were optimized in order to reproduce the results of DFT computations and experimental data on vibrational entropy, respectively. Combination of the potentials was used for on-lattice kinetic Monte-Carlo simulations. The formulated approach was tested to evaluate enthalpy of mixing, diffusion coefficients, and α−α′ phase transition boundary data for a Fe–Cr system. Importance of accounting the vibrational entropy for accurate modeling of Cr-rich clusters was demonstrated. Obtained characteristics of the α′ precipitates were in a good agreement with the experimental results. Particularly, concentration and the average radius of Cr-rich precipitates in LRP-based computations were close to APT data within the error of experimental studies.

Study on FeCr thin film for a spintronic material with negative spin polarization

Negative spin polarization materials generate a spin current whose spin direction is opposite to the magnetization direction. This characteristic is technologically important because it increases the freedom in the design of spintronic devices. In this paper, we studied FeCr from the viewpoints of band structure, atomic magnetic moments, and local atomic arrangement to explore its potentiality as a negative spin polarization material. First-principles calculations on the disordered FeCr alloy predicted a high negative spin polarization, which was attributed to the band matching of the minority spin. We experimentally fabricated Fe1-xCrx thin films in the Cr concentration (x) range from 0.2 to 0.4 by sputter deposition and demonstrated inverse magnetoresistance using current-perpendicular-to-plane giant magnetoresistance (GMR) devices, which provided evidence of negative spin polarization. The spin polarization of Fe0.7Cr0.3 estimated from the GMR device measurements was −0.28 and fell below the calculated value. Synchrotron X-ray magnetic circular dichroism measurements showed a ferrimagnetic configuration of the Fe and Cr magnetic moments. The Fe moment showed a large moment over a wide Cr concentration range, whereas the Cr moment decreased with the Cr concentration, which agreed with the calculation result. Synchrotron Mössbauer spectroscopy measurements indicated the inhomogeneity of the FeCr composition even in the as-deposited state. Fe-rich regions were formed after annealing, reflecting the phase separation nature of the Fe-Cr system. Inhomogeneity was not considered in the calculation and could be the origin of the smaller negative spin polarization observed in the experiment.

Late stage coarsening kinetics induced by a phase-dependent mobility: A phase field study of FeCr in the spinodal regime

An efficient algorithm is presented in this work to compute the late stage coarsening kinetics within the phase-field framework. Microstructures resulting from off-critical quenches over large time periods have been simulated in the FeCr system, which exhibits substantial variations in atomic mobilities between the Fe-matrix and Cr-rich precipitates. To understand the impact of such a phase-dependent mobility on the microstructure, 2D simulations have been performed in the spinodal regime using a constant, symmetric, and phase-dependent mobility. Detailed analyses of these simulations highlight the fact that the more realistic situation where atomic mobility is non-uniform induces a competition between coalescence and coarsening of precipitates. We use these simulations to shed light upon previously published numerical and experimental results.

Thermodynamic modelling to predict phase stability in BCC + B2 Al–Ti–Co–Ni–Fe–Cr high entropy alloys

This paper examines the potential of thermodynamic modelling as a simple and inexpensive means for assessing phase stability in a series of non-equiatomic high entropy alloys and compares with CALPHAD calculations to demonstrate an appropriate level of simplifying assumptions. The modelling was motivated by alloys from the Al–Ti–Co–Ni–Fe–Cr system, which were produced by iteratively following the natural compositional segregation of the two-phase BCC + B2 microstructure present in a Al2TiCoNiFeCr alloy after casting and heat treatment. This produced a range of multicomponent B2-type alloys with different volume fractions of a BCC secondary phase. The solubility limits and traditional empirical thermodynamic driving forces for phase stability were investigated to explain the formation of the two phases. Limitations of prior semi-empirical models are highlighted, with advancements demonstrated by accounting for contributions from the effect of ordering on configurational entropy, the difference in enthalpy from intermetallic compounds, and thermal influences on both entropy and enthalpy. The new models are compared against the current leading thermodynamic modelling approach, CALPHAD, with excellent correlation. This work outlines a methodology to predict and design phase constitution in future high-performance BCC + B2 alloys and, more generally, it demonstrates the value of models with temperature-dependent thermodynamic quantities for exploring new, complex compositional regions.

Model of Decomposition of Alloy with Two Magnetic Components: the BCC FeCr System

Model of Decomposition of Alloy with Two Magnetic Components: the BCC FeCr System

Ab initio simulations on the pure Cr lattice stability at 0K: Verification with the Fe-Cr and Ni-Cr binary systems

Significant discrepancies have been observed and discussed on the lattice stability of Cr between the predictions from the ab initio calculations and the CALPHAD approach. In the current work, we carefully examined the possible structures for pure Cr and reviewed the history back from how Kaufman originally determined the Gibbs energy of FCC-Cr in the 1970s. The reliability of Cr lattice stability derived by the CALPHAD and ab initio approaches was systematically discussed. It is concluded that the Cr lattice stability based on the CALPHAD approach has large uncertainty. Meanwhile, we cannot claim that the ab initio HFCC-Cr is error-free as FCC-Cr is an unstable phase under ambient conditions. The present work shows that the ab initio HFCC-Cr can be a viable scientific approach. As both approaches have their limitations, the present work propose to integrate the ab initio results into the CALPHAD platform for the development of the next generation CALPHAD database. The Fe-Cr and Ni-Cr binary systems were chosen as two case studies demonstrating the capability to adopt the ab initio Cr lattice stability directly into the current CALPHAD database framework.